1. Department of Chemistry; University of Delhi; Delhi- 110007 India
2. Present address: National Institute of Nanotechnology; University of Alberta; Edmonton- T6G 2M9 AB Canada
3. Centre for Nanoscience and Nanotechnology; Panjab University; Chandigarh- 160015 India
4. Department of Chemistry; Indian Institute of Technology, Tirupati; Tirupati- 517506, AP India
5. Applied Physical Chemistry; Heidelberg University; Im Neuenheimer Feld 253 69120 Heidelberg Germany